Composition for an injectable bone mineral substitute material

ABSTRACT

The invention refers to an injectable composition for a bone mineral substitute material, which comprises a dry powder mixed with an aqueous liquid. The powder comprsises a first reaction component comprising a calcium sulphate hemihydrate with the capability of being hardened to calcium sulphate dihydrate when reacting with said aqueous liquid; a second reaction component, which comprise a calcium phosphate with the capability of being hardened to a calcium phosphate cement when reacting with said aqueous liquid; and at least one accelerator for the reaction of said fist and/or second reaction component with said aqueous liquid. A method of producing an injectable bone mineral substitute material is also provided, wherein the composition is mixed in a closed mixing an d delivery system for delivery.

TECHNICAL FIELD

[0001] The present invention relates to an injectable composition for abone mineral substitute material with the capability of being hardenedin a body fluid in vivo. Furthermore, the invention relates to a methodof producing such a material.

BACKGROUND ART

[0002] During the last decade, the number of fractures related toosteoporosis, i.e. reduced bone mass and changes in microstructureleading to an increased risk of bone fractures, has almost doubled. Dueto the continuously increasing average life time it is estimated that by2020 people over 60 years of age will represent 25% of Europe'spopulation and that 40% of all women over 50 years of age will sufferfrom an osteoporotic fracture.

[0003] With the aim to reduce or eliminate the need for bone grafting,research has been made to find a suitable artificial bone mineralsubstitute. Presently, at least the following bone mineral substitutesare used for the healing of bone defects and bone fractures, namelycalcium sulphates, as for instance Plaster of Paris, calcium phosphates,as for instance hydroxylapatite, and polymers, as for instancepolymethylmetacrylate (PMMA).

[0004] Calcium sulphate (Plaster of Paris), CaSO₄.½H₂O, was one of thefirst materials investigated as a substitute for bone grafts. Studieshave been undertaken since 1892 to demonstrate its acceptance by thetissues and rapid rate of resorbtion It has been concluded that Plasterof Paris implanted in areas of subperiosteal bone produces no furtheruntoward reaction in the tissue than normally is present in a fracture.Regeneration of bone in the area of subperiosteal resection occursearlier than when an autogenous graft is used. Plaster of Paris does notstimulate osteogenesis in the absence of bone periosteum. The new bonegrowing into Plaster of Paris is normal bone. No side effectsattributable to the implantation of Plaster of Paris have been noted inadjacent tissues or in distant organs. However, Plaster of Paris has thedrawback of very long setting times, which constitutes problems atsurgery.

[0005] Another group of materials for substituting bone tissue infracture sites and other bone defects is calcium phosphate cements. Dueto their biocompatibility and their osteoconductivity they can be usedfor bone replacement and augmentation.

[0006] Hydroxylapatite, a crystalline substance which is the primarycomponent of bone, is mainly used as a bone substitute, but is notstrong enough for use under weight bearing conditions. Experiments haveshown that hydroxy-lapatite cement forms a stable implant in respect ofshape and volume over 12 months and has the same excellent tissuecompatibility as exhibited by commercial ceramic hydroxylapatitepreparations. Microscopic examination clearly demonstrated thathydroxylapatite cement was progressively ingrown by new bone over time.

[0007] Although the ideal is to achieve hydroxylapatite, there are alsoapatite-like calcium phosphates which can be obtained as potential bonesubstitutes. In Table 1 calcium phosphates are presented which areformed by a spontaneous precipitation at room or body temperature, aswell as the pH range, within which these components are stable. TABLE 1Calcium phosphates obtained by precipitation at room or body temperatureCa/P Formula Name pH 0.5 Ca(H₂PO₄) · H₂O MCPM 0.0-2.0 1 CaHPO₄ · 2H₂ODCPD 2.0-6.0 1.33 Ca₈(HPO₄)₂(PO₄)₄ · 5H₂O OCP 5.5-7.0 1.5 Ca₉(HPO₄)(PO₄)₅OH CDHA 6.5-9.5 1.67 Ca₅(PO₄)₃OH PHA 9.5-12 

[0008] Other calcium phosphates can be obtained by means of sintering athigh temperatures, above 1000° C. (Table 2). These calcium phosphatescan not be obtained by precipitation in room or body temperature.However, they can be mixed with an aqueous solution alone or incombinations with other calcium phosphates to form a cement-like pastewhich will set with time. TABLE 2 Components forming calcium phosphatecements Ca/P Compound Formula Name 1.5 α-tricalcium α-Ca₃(PO₄)₂ α-TCPphosphate 1.5 β-tricalcium β-Ca₃(PO₄)₂ β-TCP phosphate 1.67 SinteredCa₁₀(PO₄)₆(OH)₂ SHA hydroxylapatite 2.0 Tetracalcium Ca₄(PO₄)₂O TTCPphosphate

[0009] Bone mineral substitute materials can be used for preparing apaste which can be injected directly into a fracture site. The paste isinjected into the void in the bone and, upon hardening, an implant isobtained which conforms to the contours of the gap and supports thecancellous bone. Both calcium sulphate and hydroxylapatite materialshave been extensively investigated as a possible alternative toautogenous bone grafts to help restore osseous defects of bone andfixation of bone fracture.

[0010] In this connection it is important that a complete stability isobtained as quickly as possible during or after surgery in order toprevent motions at site of healing. This especially applies tofractures, but also when filling of a bone cavity or replacing bone lostduring tumor removal the healing is inhibited by movements and theingrowth of new bone is prevented. Thus, the injected material must curefast and adhere firmly to the bone tissue.

[0011] It is also of importance that the hardened material is so similarin structure to the bone so that it can be gradually resorbed by thebody and replaced by new bone growth. This process can be facilitated ifthe hardened cement is provided with pores, which can transportnutrients and provide growth sites for new bone formation.

[0012] M. Bohner et al. disclosed at the Sixth World Bio-materialsCongress Transactions (May 15-20, 2000) a method to obtain an openmacroporous calcium phosphate block by using an emulsion of ahydrophobic lipid (oil) in an aqueous calcium phosphate cement paste oran emulsion of an aqueous calcium phosphate cement paste in oil. Aftersetting, the cement block was sintered at 1250° C. for 4 hours.Likewise, CN 1193614 shows a porous calcium phosphate bone cement forrepairing human hard tissue. The cement contains pore-forming agentwhich may be a non-toxic surfactant, or a non-toxic slightly solublesalt, acidic salt and alkaline salt.

[0013] Studies have also been made on mixtures of the above mentionedbone mineral substitute materials. In U.S. Pat. No. 4,619,655 isdisclosed a bone mineral substitute material comprising a mixture ofPlaster of Paris, i.e. calcium sulphate hemihydrate, and calciumphosphate ceramic particles, preferably composed of hydroxylapatite, ortricalcium phosphate or mixtures thereof. According to U.S. Pat. No.4,619, 655 tests show that when alloplasts composed of 50/50 mixtures ofhydroxylapatite/Plaster of Paris were implanted into experimentallycreated defects in rat mandible, the Plaster of Paris was completelyresorbed within a few weeks and replaced by connective tissue. Thehydroxylapatite was not resorbed and some particles were eventuallycompletely surrounded by bone. It was therefore concluded that thePlaster of Paris acted as a scaffold for the incorporation ofhydroxylapatite into bone.

[0014] A recent study presented on the “Combined Orthopaedic ResearchSocieties Meetings”, Sep. 28-30, 1998, Hamamatsu, Japan, also showsadditional tests relating to mixtures of Plaster of Paris andhydroxylapatite. According to this study a combination ofhydroxylapatite particles and Plaster of Paris had a viscosity whichallowed an easy placement of the implant material and preventedmigration of hydroxylapatite particles into surrounding tissues duringand after implantation. The experiments showed that Plaster of Paris wasabsorbed in relatively short time, was easily manipulated withhydroxylapatite particles, and did not interfere with the process ofbone healing.

[0015] WO 9100252 shows a composition which is capable of hardening inblood within about 10-45 min. The composition comprises essentiallycalcium sulphate hemihydrate with small amounts of calcium sulphatedihydrate. Organic and inorganic materials, such as hydroxylapatite, canalso be included in the composition. After hardening, particles ofhydroxylapatite are obtained within a calcium sulphate cement. Thecalcium sulphate cement is dissolved rapidly by aqueous body fluidswithin four weeks, leaving solid particles of hydroxylapatite.

[0016] Likewise, such particles of hydroxylapatite within a calciumsulphate cement are obtained by the method of WO 9117722. Thecomposition for use as an animal implant comprises calcium sulphatehemihydrate, calcium phosphate, and sodium sulphate. The calciumphosphate is hydroxylapatite and the sodium sulphate enables thecomposition to be used in the presence of blood or other body fluids.

SUMMARY OF THE INVENTION

[0017] The object of the invention is to provide an injectablecomposition for a bone mineral substitute material with the capabilityof being hardened in a body fluid in vivo, which hardens during surgerywith accompanying early control of fracture fragment movement as well asprovides a stable lasting implant over a year with high mechanicalstrength, and which during this later period presents a porous as wellas irregular structure for bone ingrowth.

[0018] A further object of the present invention is to provide such animproved injectable bone mineral substitute for filling defects inosteoporotic bone and for additional fracture fixation in substantiallycancellous bone which does not exhibit the drawbacks of high viscosityat delivery and low fracture toughness.

[0019] Still another object of the invention is to provide an injectablebone mineral substitute having excellent biocompatibility, favorablebiological and rheological properties. The bone mineral substituteshould also be biodegradable and be possible to sterilize by radiationor gas without suffering a significant deterioration in properties.

[0020] In order to achieve these objects the injectable compositionaccording to the invention has been given the characterizing features ofclaim 1.

[0021] According to the invention a composition is provided whichcomprises two types of bone cement materials, which both are subjectedto a hardening reaction in contact with water.

[0022] A cement of hardened calcium sulphate (gypsum) will remain set ina dry environment. In a wet environment, such as in a Body SimulatedSolution, this material will immediately start to disintegrate. Thus, animplanted material with reduced strength will be obtained in the body.The solid material obtained will start to degrade, eventually within 1-2days.

[0023] On the other hand, in order to induce a setting (hardening)reaction in a Body Simulated Solution or in a body with its blood,saline can be used. By using saline a setting will be obtainedimmediately under any conditions, but the implant obtained will stilldegrade quite rapidly.

[0024] The second reaction, in which a calcium phosphate is hardened(cemented) to a calcium phosphate cement in the presence of water, willtake longer time—about 18 h or more—in order to set to a high strengthmaterial. During this period of time the already set sulphate willconfer an initial strength to the implant, and when the setting reactionof tricalcium phosphate to a high strength material is completed, afinal strength will be obtained, which lasts for months or years.

[0025] In this connection the term “calcium phosphate cement” refers tothe recognized definition (S. E. Gruninger, C. Siew, L. C Chow, A.O'Young, N. K. Tsao, W. E. Brown, J. Dent. Res. 63 (1984) 200) of areaction product of a powder or a mixture of powders which—after mixingwith water or an aqueous solution to a paste—at a temperature aroundroom temperature or body temperature react with the formation of aprecipitate, which contains crystals of one or more calcium phosphatesand which sets by the entanglement of the crystals within theprecipitate. Thus, different calcium phosphate products (calciumphosphate cements) can be obtained during the setting reaction independence on the component(s) of the powders used for the pasteinventive injectable composition for a bone mineral substitute material.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The invention will now be explained in more detail, referencebeing made to the accompanying drawings, in which

[0027]FIG. 1 shows the effects of α-tricalcium phosphate on compressivestrength;

[0028]FIG. 2 shows the effects of the content of calcium sulphatedihydrate on the injection time; and

[0029]FIG. 3 shows the effects of the water content and the content ofcalcium sulphate dihydrate on the setting time.

[0030] In order to accomplish an injectable bone mineral substitutematerial having improved characteristics, tests were made with theobject to evaluate the effects of particle size, water content andaccelerator on the viscosity, setting time and porosity of theinjectable bone mineral substitute material of the invention.

[0031] The inventive injectable composition for a bone mineralsubstitute material comprises a dry powder mixed with an aqueous liquid.A main requirement on such a material is its setting time, which shouldbe within 5-12 minutes. Additionally, the viscosity of the materialshould be adapted to render it injectable into the bone for 1-5 minutesafter the beginning of the mixing procedure.

[0032] The evaluated materials comprised calcium sulphate hemihydrate,also known as Plaster of Paris. It was found that the addition of asmall amount of finely ground already reacted calcium sulphatedihydrate, CaSo₄.2H₂O, had a decisive impact on the setting time and theinjectable time of the bone mineral substitute. Due to the addition ofan accelerator the setting time period was considerably shortened whilethe injectable time was still long enough to make it possible to injectthe material of the invention into e.g. a bone cavity. It is assumedthat other accelerators and mixtures of accelerators may be used, e.g.starch, mixtures of calcium sulphate dihydrate and lignosulphate,calcium sulphate dihydrates having composite coatings, etc.

[0033] Those reactions which forms hydroxylapatite, i.e. precipitatedhydroxylapatite (PHA) or calcium deficient hydroxylapatite (CDHA), canbe classified into three groups. The first group consists of calciumphosphates, which are transformed into hydroxylapatite by a hydrolysisprocess in an aqueous solution (eq. 1-5).

5Ca(H₂PO₄).H₂O→Ca₅(PO₄)₃OH+7H₃PO₄+4H₂O  (1)

5CaHPO₄.2H₂O→Ca₅(PO₄)₃OH+2H₃PO₄+9H₂O  (2)

5Ca₈H₂(PO₄)₆.5H₂O→8Ca₅(PO₄)₃OH+6H₃PO₄+17H₂O  (3)

5Ca₃(PO₄)₂+3H₂O→3Ca₅(PO₄)₃OH+H₃PO₄  (4)

3Ca₄(PO₄)₂O+3H₂O→2Ca₅(PO₄)₃OH+Ca(OH)₂  (5)

[0034] Precipitated hydroxylapatite is the least soluble calciumphosphate at pH over 4,2. This means that any other calcium phosphatepresent in an aqueous solution at this pH range will tend to dissolve,with the precipitation of PHA as a product. This hydrolysis process(Ca(OH)₂—H₃PO₄—H₂O) is very slow due to a decrease in supersaturation asthe reaction proceeds.

[0035] The only calcium phosphate which can react via a hydrolysisprocess to an apatite without the formation of sub-products isα-tricalcium phosphate (eq. 6), and the apatite formed in this reactionis a calcium deficient hydroxylapatite.

3α-Ca₃(PO₄)₂+H₂O→Ca₉(HPO₄)(PO₄)₅OH  (6)

[0036] The second group of reactions to a hydroxylapatite, i.e.precipitated hydroxylapatite (PHA) or calcium deficient hydroxylapatite(CDHA), is the combinations between TTCP and other calcium phosphates.TTCP is the only calcium phosphate with Ca/P ratio above 1.67. Thus,this substance can be mixed with other calcium phosphates with lowerCa/P ratio to obtain PHA or CDHA without the formation of acids or basesas by-products. Theoretically, any calcium phosphate more acid than PHAcan react directly with TTCP to form HA or CDHA according to thefollowing chemical reactions.

7Ca₄(PO₄)₂O+2Ca(H₂PO₄)₂.H₂O→6Ca₅(PO₄)₃OH+3H₂O  (7)

2Ca₄(PO₄)₂O+Ca(H₂PO₄)₂.H₂O→Ca₉(HPO₄)(PO₄)₅OH+2H₂O  (8)

Ca₄(PO₄)₂O+CaHPO₄.2H₂O→Ca₅(PO₄)₃OH+2H₂O  (9)

3Ca₄(PO₄)₂O+6CaHPO₄.2H₂O→2Ca₉(HPO₄)(PO₄)₅OH+13H₂O  (10)

Ca₄(PO₄)₇O+CaHPO₄→Ca₅(PO₄)₃OH  (11)

3Ca₄(PO₄)₂O+6CaHPO₄→2Ca₉(HPO₄)(PO₄)₅OH+H₂O  (12)

3Ca₄(PO₄)₂O+Ca₈H₂(PO₄)₆.5H₂O→4Ca₅(PO₄)₃OH+4H₂O  (13)

3Ca₄(PO₄)₂O+3Ca₈H₂(PO₄)₆.5H₂O→4Ca₉(HPO₄)(PO₄)₅OH+14H₂O  (14)

Ca₄(PO₄)₂O+2Ca₃(PO₄)₂+H₂O→Ca₅(PO₄)₃OH  (15)

[0037] In equations (7) and (8) DCPD is formed as an intermediatereaction product, but with PHA or CDHA at the end of the reaction.Reactions (13), (14), and (15) are all very slow. However, by using theformulas (9)-(12) it is possible to produce a cement which sets andhardens with time at room or body temperature and at a neutral pH.

[0038] It is also possible to form PRA as the final hardened product byusing mixtures of calcium phosphates with a Ca/P ratio of less than1.67. This is accomplished by using additional calcium sources, such asCa(OH)₂ or CaCO₃, instead of TTCP. One example is the reactionβ-TCP+DCPD+CaCO₃→PHA. Initially formed crystals of PHA from a reactionbetween CDPD and CaCO₃ function as binders between β-TCP particles. WhenDCPD is consumed the reaction continues between the remaining calciumcarbonate and β-TCP with the formation of PHA. However, it seems thatthe latter process has a detrimental effect on the mechanical strengthof the cement.

[0039] It is preferred that the calcium phosphate with the capability ofbeing hardened to a calcium phosphate cement when reacting with anaqueous liquid is tricalcium phosphate (TCP), tetracalcium phosphate(TTCP), anhydrous dicalcium phosphate, monocalcium phosphate monohydrate(MCPM), dicalcium phosphate dihydrate (DCPD), or octo-calcium phosphate(OCP). Preferably, the calcium phosphate is α-tricalcium phosphate.

[0040] In order to confer an initial strength to a bone mineralsubstitute material the calcium sulphate hemihydrate in the compositionaccording to the invention should comprise 2-80 wt %, preferably 10-30wt % of the dry powder to be mixed with an aqueous liquid. Likewise, thecalcium phosphate to be hardened to a calcium phosphate cement shouldcomprise 10-98 wt %, preferably 70-90 wt % of the dry powder. In thecomposition, the aqueous liquid should comprise between 0.1 and 2 ml,preferably between 0.5 and 1 ml per gram powder.

[0041] By preferably using particulate reaction components in theinventive composition, a high strength implant material will be obtainedinitially. The fast setting calcium sulphate material will be formedwithin a block of a slow setting material, i.e. the calcium phosphatecement. Thus, when initial strength decreases the second strengthincreases, and its final strength will be maintained within the body.Pores, holes and cavities will gradually be formed as the sulphatedegrades, which acts like lacuna, and the finally set and hardenedimplant of a high strength material will look like a normal bone.

[0042] Both reactions in the inventive composition can be controlled byincluding an accelerator or a retarder. By using seed particles, theprocesses can be accelerated.

[0043] If such an accelerator is added, the calcium sulphate hemihydratewill set rapidly, i.e. within 10 min. Particulate calcium sulphatedihydrate is a suitable accelerator for this reaction, the particle sizebeing less than 1 mm. A more efficient reaction is obtained if theparticulate calcium sulphate dihydrate has a particle size of less than150 μm, preferably less than 100 μm, and most preferable less than 50μm. The particulate calcium sulphate dihydrate should comprise between0.1 and 10 wt %, preferably between 0.1 and 2 wt % of the calciumsulphate hemihydrate which is to react with an aqueous liquid. Theaccelerator should be adapted so that a set material is obtained within15 min, preferably within 8 min which has a threshold strength of about30 MPa in a clinical situation. Preferably, the partitulate calciumsulphate dihydrate is α-calcium sulphate dihydrate.

[0044] The second reaction of a calcium phosphate to a calcium phosphatecement sets slowly, but can be controlled to set within 18 h as a bonemineral substitute material with a strength of about 30 MPa. This can beaccomplished by adding hardened particulate calcium phosphate cement tothe inventive composition. The hardened calcium phosphate cement can behydroxylapatite (HA), preferably precipitated hydroxylapatite (PHA),tricalcium phosphate (TCP), or a mixture thereof. It should have a Ca/Pratio between 1.5 and 2. The particulate calcium phosphate cement shouldhave a particle size which is less than 20 μm, preferably less than 10μm and comprise between 0.1 and 10 wt %, preferably between 0.5 and 5 wt% of the calcium phosphate which is to react with an aqueous liquid.

[0045] The reaction of calcium phosphate to a calcium phosphate cementcan also be accelerated by a phosphate salt, for example disodiumhydrogen phosphate (Na₂HPO₄), which is dissolved in the aqueous liquid.In this case, the accelerator should be present in the aqueous liquid atconcentrations of 0.1-10 wt %, preferably 1-5 wt %.

[0046] The two types of accelerator for the reaction of calciumphosphate to calcium phosphate cement can be used either separately orin combination.

[0047] In the composition according to the invention the aqueous liquidcan be distilled water or a balanced salt solution, such as PBS, PBSS,GBSS, EBSS, HBSS, or SBF.

[0048] The injectability of the composition according to the inventioncan be improved in several ways. It has surprisingly been shown that apl reducing component can be added to the inventive composition, theinjectability thereof being improved. Such a pH reducing component isfor example ascorbic acid or citric acid. These acids are included inthe sterile liquid or the sterile powder of the composition in amountsof 0.1-5 wt %, preferably 0.5-2 wt %.

[0049] Another way to improve the injectability of the composition is toadd a biologically compatible oil. The concentration of the oil shouldbe between 0.1 and 5 wt %, preferably between 0.5 and 2 wt %. A suitableoil to be used in the inventive composition is vitamin E. The oil caneither be intermixed with the sterile powder or included in the sterileliquid of the composition.

[0050] As stated above, the addition of a small amount of alreadyreacted calcium sulphate dihydrate had an effect on the injectable timeof the bone mineral substitute. Thus, by replacing some of thenon-reacted calcium sulphate hemihydrate with reacted calcium sulphatedihydrate, the injectability of the composition could be improved. Asmuch as 95% of the hemihydrate can be replaced. Preferably, 50-90% ofthe hemihydrate is replaced by the dihydrate, most preferred 80-90%.

[0051] In order to further improve the bone mineral substitute materialobtained with the inventive composition it ismpossible to furtherinclude additional substances, e.g. growth factors, anti-cancersubstances, antioxidants and/or antibiotics, etc. Antibiotic containingbone cement is already known and it has been shown that addition ofantibiotics to synthetic hydroxylapatite and cancellous bone releasessaid antibiotics in a concentration sufficiernt for treating boneinfections when said substances are administered into the bone.

[0052] An efficient mixing system must be available in order to preparethe composition according to the invention. The mixing can take place ina conventional cement mixing system and the composition is injected bymeans of a convenient delivery system. The mixing container ispreferably of that type which can suck the aqueous component into thepowder component (German Patent 4409610). This Prepack™ system is aclosed mixing system for delivery in combination with prepackedcomponents in a flexible foil bag. Other mixing devices can of coursealso be used, for example two interconnected soft bags which can beadapted to a delivering cylinder.

[0053] The formation of air bubbles in the composition, which caninterfere with the hardening reaction of the calcium sulphatehemihydrate and result in a decreased initial mechanical strength of theimplanted material during surgery, can be prevented by mixing thecomposition under conditions of subatmospheric pressure, e.g. in vacuo.However, an atmospheric pressure can also be used. Preferably, thepowder component of the composition is sterilized by means of radiationbefore it is mixed with the sterile liquid component.

EXAMPLES

[0054] The invention will now be further described and illustrated byreference to the following examples. It should be noted, however, thatthese examples should not be construed as limiting the invention in anyway.

Comparative Example 1

[0055] As a control test the injectable time and the setting time ofpure calcium sulphate hemihydrate were determined to be more than 10 and20 minutes, respectively.

Comparative Example 2

[0056] As a second control test the injectable time and the setting timeof a mixture of calcium sulphate hemihydrate, and hydroxylapatite werealso determined to be more than 10 and 20 minutes, respectively.

Comparative Exawple 3

[0057] The injectable time (IT) and the setting time (SI) were studiedfor the first reaction of a calcium sulphate hemihydrate to calciumsulphate dihydrate in the presence of a passive additive. Twentydifferent mixtures of calcium sulphate hemihydrate, hydroxylapatite (HA)and accelerator (Acc) were evaluated, which had different ratios ofhydroxylapatite and accelerator, see Table 3. The setting time wasdetermined by a mechanical test. A metallic rod having a weight of 23 g,a diameter of 10 mm and a length of 35 mm was dropped from a height of35 mm. The time when the rod did not leave any mark on the sample wasregistered as the setting time. TABLE 3 TEST CASO₄ HA HA ACC IT SI NO.(G) (G) (%) (%) (MIN) (MIN) 1 32 4 10 10 1.5 3.0 2 28 8 20 10 1.5 4.0 324 12 30 10 1.5 4.0 4 20 16 40 10 2.0 6.0 5 16 20 50 10 1.5 6.0 6 34 410 5 2.0 5.0 7 30 8 20 5 1.5 5.0 8 26 12 30 5 2.5 7.0 9 22 16 40 5 2.57.5 10 18 20 50 5 2.0 7.0 11 35 4 10 2.5 1.5 5.0 12 31 8 20 2.5 1.5 5.013 27 12 30 2.5 2.0 7.5 14 23 16 40 2.5 2.5 7.5 15 19 20 50 2.5 2.5 10.016 35.6 4 10 1 2.5 7.0 17 31.6 8 20 1 3.0 9.0 18 27.6 12 30 1 3.5 10.519 23.6 16 40 1 4.0 13.0 20 19.6 20 50 1 4.0 14.5

Example 1

[0058] Different bi-phasic injectable cements were produced, which werebased on α-tricalcium phosphate and α-calcium sulphate hemihydrate.

[0059] The mechanical strength of each cement produced was evaluatedwith time at 10 hours, 24 hours, 3 days, and 14 days after mixing of thecement with water. The evaluation was performed at the time periodsgiven by means of a cylindrical specimen (d=6 mnm, h=12 mm) that hadbeen immersed in a physiological saline solution of 37° C. The resultsare shown in Table 4 below. TABLE 4 Amount Compressive CompressiveCompressive Compressive of strength strength strength strength α-TCP 10h 24 h 3 d 14 d (wt %) (MPa) ± S.D. (MPa) ± S.D. (MPa) ± S.D. (MPa) ±S.D. 0 11 3.63 7.64 1.41 12.99 2.66 9.66 3.2 20 1.01 0.39 1.69 0.49 3.990.35 5.36 0.33 40 0.68 0.25 5.08 1.66 8.82 1.2 9.82 1.86 60 3.58 1.025.1 0.91 15.73 5.24 14.13 1.42 80 5.31 1.03 10.72 0.69 21.8 3.41 23.923.06 100 6.24 1.48 22.37 6.34 37.99 4.74 33.98 10.37

Example 2

[0060] The compressive strength was further tested with reference toα-TCP containing less than 20 wt % calcium sulphate hemihydrate (CSH).(CSH was obtained from Bo Ehrlander AB, Gothenborg, Sweden.)

[0061] The two powders were mixed together mechanically during 5 min.Then, the liquid was added to the powder at a liquid to powder (L/P)ratio of 0.32 ml·g⁻¹. The liquid contained 2.5 wt % Na₂HPO₄ as anaccelerator.

[0062] Moulds were then filled and immersed in a saline solution (0,9%)at 37° C. for 7 days. The results are shown in Table 5 below and in FIG.1.

[0063] As seen in FIG. 1, the compressive strength was dristivallyincreased when the α-TCP content exceeded 80 wt %. TABLE 5 CompressiveStandard No. of Content of strength Deviation samples CSH (wt %) (MPa)(MPa) tested  0 62.62 7.98 7  5 34.80 9.65 7 10 23.54 10.37  8 15 22.455.12 10 

Example 3

[0064] During each of the two setting reactions, crystals are formedwhen calcium sulphate hemihydrate and calcium phosphate, respectively,react with water in the setting reactions. Initially, crystal nuclei arecreated and the final crystal structure is then formed by growth fromthe nuclei. By adding already formed crystals of set material, thenucleation step in the setting process is already completed, which willdecrease the time needed to crystallize the material and make it hard.The crystals will grow directly from particles of added calcium sulphatedihydrate and hydroxylapatite, respectively. Thus, these added particlesof set material will act as accelerators in the setting reactions.

[0065] The staller size of accelerator particles added to the material,the more efficient accelerating effect will be obtained because thecrystals will grow from the surface of the particles. It the acceleratorparticles are small, then the surface of the particles will be large perunit of weight.

[0066] When α-CaSO₄.2H₂O is used as an accelerator it will be moreefficient than β-CaSO₄.2H₂O when α-CaSO₄.½H₂O is used as the maincomponent of the material. This could be explained by the crystal shapedifference between the two forms of the calcium sulphate. Since thecrystals are growing directly from the particle surface of theaccelerator, the reaction proceeds faster if the accelerator crystalshave exactly the same shape as the crystals that are forming from themain component of the material.

Example 4

[0067] The effects of the content of calcium sulphate dihydrate on theinjection time is shown in FIG. 2. In this case the liquid/powder (L/P)ratio is 0.4 ml/g. The limit of injection time was defined when the loadreached N, which is comparable to the highest force by hand at whichinjection was possible.

Example 5

[0068] The effects of the water content and the content of calciumsulphate dihydrate on the setting time is shown in FIG. 3, wherein L/Pis the liquid-powder ratio (ml/g). The setting time was measured byusing Gilltnore Needles according to ASTM Standard C266.

Example 6

[0069] In the inventive composition, the form of the calcium sulphatehemihydrate is of importance. α-Calcium sulphate hemihydrate(α-CaSO₄.½H₂O) is advantageous to use because of its mechanicalstrength. α-CaSO₄.½H₂O has a compressive strength of 40.4 MPa comparedwith 14 MPa for β-α-CaSO₄.½H₂O.

Example 7

[0070] Biodegradation of the Calcium Sulphate with Hydroxylapatite BoneSubstitute in vitro and in vivo.

[0071] The degradation rate of calcium sulphate with 40 wt %hydroxylapatite was investigated. The material was placed in a SimulatedBody Fluid as well as muscle pockets in rats. The mechanical strengthand size of the block obtained were investigated with time as abiodegradation index.

[0072] Mechanical Testing

[0073] Compressive strength testing was performed using an MTS andInstron 8511.20 testing equipment. After harvesting the materials, thesamples were directly placed between self-levelling platens andcompressed at 1 mm min⁻¹ until failure at room temperature.

[0074] Volume Measurements

[0075] After the material harvesting, a caliper measured the volume ofthe block of material.

[0076] In Vitro Study

[0077] Cements of calcium sulphate or calcium sulphate withhydroxylapatite were prepared by mixing with distilled water at L/Pratio of 0.25 ml/g. After mixing the cement was injected into a PFTEmould and allowed to set. The samples were 4 mm in diameter and 8 mm inlength. Six cylindrical samples were placed in a Simulated Body Fluid,and the liquid was changed every day. After one week the samples weredirectly placed between self-levelling platens and subjected tocompressive strength testing until failure at room temperature.

[0078] In Vivo Study

[0079] Materials Preparation

[0080] Calcium sulphate hemihydrate (CaSO₄.½H₂O) was mixed with 40 wt %hydroxylapatite powder (Ca₁₀(PO₄)₆(OH)₂; HA). The mixture of POP-HA wassintered and quenched in air. An accelerator (a calcium sulphate) wasadded at 0.4 wt % to the POP-HA, and the dry powder material wassterilized by gamma-irradiation.

[0081] A cement was prepared by mixing the powder with distilled waterat a L/P ratio of 0.25 ml/g. Materials were prepared, which containedcalcium sulphate or calcium sulphate+hydroxylapatite. After mixing, thecement was injected into a PFTE mould and allowed to set. The sampleswere cylindrical with diameter of 4 mm and height of 8 nm. Once set, thesamples are inserted into muscle pockets of rats.

[0082] Animals

[0083] Sprague˜Dawley rats weighing around 200 g were used and kept inanimal facilities for 1 week before use. The animals were fed a standardlaboratory diet. All rats were anesthetized with peritoneal injectionsof 0.5-0.6 ml of a solution containing 1 ml pentobarbital (60 mg/ml), 2ml diazepam (5 mg/ml), and 1 ml saline (0.15 M). The implants wereinserted in muscles of the rats. Nine rate were used for each periodstudied. The rats were killed by a peritoneal injection of an overdoseof pentobarbital at 1 or 4 weeks after implantation.

[0084] Results

[0085] After one week of incubation the mechanical strength was recordedof the cylindrical samples placed in the Simulated Body Fluid or musclespockets in rats, respectively. The mechanical strength of the materialshad decreased from 35 Mpa to about 5 Mpa both in vitro as well as invivo. The volume of remaining block was only ⅓ to {fraction (1/10)} ofthe original block volume (Table 5).

[0086] After 4 weeks of incubation, the mechanical strength of thematerials had totally disappeared, and the rods of calcium sulphate werealmost completely absorbed. The calcium sulphate with hydroxylapatitewas still present but totally deformed, and the material was surroundedby normal soft tissue. The tissue also penetrated into the materials.Furthermore, the mass of remaining material was larger than the originalblock implanted.

[0087] Table 6 below shows the volume of remaining cylinder material(Mean±SE) in rat muscles after an incubation of 1 or 4 weeks. Theoriginal volume of the cylinder material was 100 mm³. Statistic analysiswas performed by using the one way ANOVA method and Student's t-test.All results obtained exhibited a high statistical significance(p<0.0001). TABLE 6 1 week incubation 4 weeks incubation No. of VolumeNo. of Volume Material samples (mm³) samples (mm³) PoP 9 31.7 ± 3.1  91.9 ± 1.5 PoP + HA 9 6.1 ± 1.5 8 159.4 ± 21.7  PoP + HA + 8 9.1 ± 2.0 8196.0 ± 17.9  Vitamin E

[0088] The implanted material comprising calcium sulphate andhydroxylapatite was rapidly degraded within one week in both SimulatedBody Fluid and in rats. The rate of degradation was the same inSimulated Body Fluid or muscles pockets indicating that only one methodis needed in order to demonstrate the degradation rate.

[0089] In conclusion, tests of the combined sulphate and phosphatematerial exhibit biodegradation in vitro and in vivo as well ashardening of both components with good results with reference toinjectability and setting.

1. An injectable composition for a bone mineral substitute material withthe capability of being hardened in a body fluid in vivo, whichcomprises a dry powder mixed with an aqueous liquid, characterized inthat said dry powder comprises a first reaction component comprising acalcium sulphate hemihydrate with the capability of being hardened tocalcium sulphate dihydrate when reacting with said aqueous liquid; asecond reaction component, which comprises a calcium phosphate with thecapability of being hardened to a calcium phosphate cement when reactingwith said aqueous liquid; and at least one accelerator for the reactionof said first and/or second reaction component with said aqueous liquid.2. A composition as in any of claims 1-3, characterized in that saidfirst and/or said second reaction component is in particulate form witha particle size of 1-100 μm, preferably 1-10 μm.
 3. A composition as inclaim 1, characterized in that said calcium sulphate hemihydrate isα-calcium sulphate hemihydrate.
 4. A composition as in any of claims1-3, characterized in that said first reaction component comprises 2-80wt %, preferably 10-30 wt % of said dry powder.
 5. A composition as inclaim 1, characterized in that said second reaction component isselected from the group comprising tricalcium phosphate (TCP),tetracalcium phosphate (TTCP), anhydrous dicalcium phosphate,monocalcium phosphate maonohydrate (MCPM), dicalcium phosphate dihydrate(DCPD), and octocalcium phosphate (OCP).
 6. A composition as in claim 5,characterized in that said tricalcium phosphate is α-tricalciumphosphate.
 7. A composition as in any of claims 1-2 or 5-6,characterized in that said second reaction component comprises 10-98 wt%, preferably 70-90 wt % of said dry powder.
 8. A composition as inclaim 1, characterized in that said at least one accelerator for thereaction of said first reaction component with said aqueous liquid isparticulate calcium sulphate dihydrate.
 9. A composition as in claims 8,characterized in that said particulate calcium sulphate dihydrate isα-calcium sulphate dihydrate.
 10. A composition as in claim 8 or 9,characterized in that said particulate calcium sulphate dihydrate has aparticle size of less than 1 mm.
 11. A composition as in claim 10,characterized in that said particulate calcium sulphate dihydrate has aparticle size of less than 150 μm, preferably less than 50 μm.
 12. Acomposition as in any of claims 8-11, characterized in that saidparticulate calcium sulphate dihydrate comprises between 0.1 and 10 wt%, preferably between 0.1 and 2 wt % of said first reaction component.13. A composition as in claim 1, characterized in that said at least oneaccelerator for the reaction of said second reaction component with saidaqueous liquid is particulate calcium phosphate cement.
 14. Acomposition as in claim 13, characterized in that said particulatecalcium phosphate cement has a Ca/P ratio between 1.5 and
 2. 15. Acomposition as in claim 13 or 14, characterized in that said particulatecalcium phosphate cement is hydroxylapatite (HA), tricalcium phosphate(TCP), or a mixture thereof.
 16. A composition as in claim 15,characterized in that said hydroxylapatite is precipitatedhydroxylapatite (PHA).
 17. A composition as in any of claims 13-16,characterized in that said particulate calcium phosphate cement has aparticle size which is less than 20 μM, preferably less than 10 μm. 18.A composition as in any of claims 13-17, characterized in that saidparticulate calcium phosphate cement comprises between 0.1 and 10 wt %,preferably between 0.5 and 5 wt % of said second reaction component. 19.A composition as in claim 1, characterized in that said aqueous liquidcomprises destilled water or a balanced salt solution.
 20. A compositionas in claim 1 or 19, characterized in that said at least one acceleratorfor the reaction of said second component with said aqueous liquid isdissolved in said aqueous liquid.
 21. A composition as in claim 20,characterized in that said accelerator is disodium hydrogen phosphate(Na₂HPO₄).
 22. A composition as in claim 20 or 21, characterized in thatsaid accelerator comprises 0.1-10 wt %, preferably 1-5 wt % of saidaqueous liquid.
 23. A composition as in claim 1 or 19, characterized inthat said aqueous liquid comprises between 0.1 and 2 ml, preferablybetween 0.5 and 1 ml per gram of said powder.
 24. A composition as inany of claims 1-23, characterized in that up to 95%, preferably between80 and 90%, of said calcium sulphate hemihydrate is replaced by hardenedcalcium sulphate dihydrate in order to improve the injectabilitythereof.
 25. A composition as in any of claims 1-23, characterized inthat it further comprises a biologically compatible oil in order toimprove the injectability thereof.
 26. A composition as in claim 1,characterized in that said biologically compatible oil is vitamin E. 27.A composition as in claim 26 or 27, characterized in that saidbiologically compatible oil comprises between 0.1 and 5 wt %, preferablybetween 0.5 and 2 wt %.
 28. A composition as in any of claims 1-23,characterized in that it further comprises a pH reducing component inorder to improve the injectability thereof.
 29. A composition as inclaim 28, characterized in that said a pH reducing component is ascorbicacid or citric acid.
 30. A composition as in claim 28 or 97,characterized in that said pH reducing component comprises between 0.1and 5 wt %, preferably between 0.5 and 2 wt %.
 31. A composition as inclaim 1, characterized in that said dry powder is sterile.
 32. Acomposition as in claim 1, characterized in that it further comprisesbiologically active substances, such as growth factors and/oranti-cancer substances and/or antibiotics and/or antioxidants. 33.Method of producing an injectable bone mineral substitute material,wherein a composition as in any of claims 1-32 is mixed in a closedmixing and delivery system, preferably under conditions ofsubatmospheric pressure.